Adhesives

An adhesive is any substance that creates a bond between either the surface of a material and the adhesive itself or between the surfaces of two different materials. The purpose of this bond is to resist any forces that try to separate the objects that have been bonded together. Common forms that adhesives may take are glue, paste, laminates, cement, mortar, or the adhesive backing of tapes and seals. Adhesives are an attractive option for binding because they offer the ability to easily bind different materials together, are generally inexpensive, and efficiently distribute stress over a large area.

Adhesive Testing:

The purpose of adhesive testing is to determine whether or not a substance is suitable for a particular application. Many adhesives are subject to a wide range of forces and stresses during their lifetime but will most commonly experience shear, tension, peel, or any combination of these. Testing adhesives using these forces will reveal its strength which may be summarized by its tensile strength, shear strength and peel strength which are each effectively the point at which the adhesive fails due to those forces. During the adhesive testing one of three failures will occur. The adhesive may fail in cohesion, where the actual adhesive is ruptured, or in adhesion, where the adhesive is separated cleanly from the bonded surface, or a mixture of both adhesion and cohesion.

Adhesive Testing Specimens:

The forms the adhesive may take are the bond between two flexible substrates, two rigid substrates, or a flexible and rigid substrate. These three forms generally require universal test machines to determine the strength of the adhesive.

Common Adhesive Test Methods:

There are many different methods for the testing of adhesives but the most popular involve tensile, shear and peel forces. The tensile testing of adhesives is generally performed for two different situations: the bonding of two rigid substrates and the bonding of a flexible and rigid substrate.

Tensile testing two rigid substrates bonded together with an adhesive requires that both substrates be gripped properly and pulled apart at a constant rate until either the adhesive or the substrates fail. For a flexible to rigid bonded assembly a loop tack test is used for the testing of tensile properties. The rigid substrate is gripped at the bottom while the flexible substrate is formed into a loop with the adhesive facing out and gripped from above. The flexible surface is then adhered to the rigid surface and then pulled straight up so that the two materials are separated cleanly.

Shear testing of adhesives generally involves a single or double lap joint and a universal shear testing machine. The adhesive is applied to the joint between the two materials so that one continuous length is formed. The two ends of the new sample are then placed into their respective grips in the shear tester. The tensile shear tester is then used to apply a load to the joint until it is ruptured in shear.

A peel test is used for the testing of the bond of the adhesive between a flexible substrate and rigid substrate or two flexible substrates. When a flexible material is bonded to a rigid material either a 90 or 180 degree peel test will be used. These two tests involve the flexible substrate to be bent to the required angle and then pulled away from the rigid substrate breaking the adhesive bond between the two. If two flexible substrates are bonded together the common test method is the T peel test in which the sample is loaded into a tensile tester in a manner that causes one substrate to point upwards and the other to point downwards effectively forming a “T” at the joint. The peel tester then pulls the two substrates away from each other destroying the bond in the process.

Overall, adhesive material testing is a valuable tool for ensuring the quality and reliability of adhesive bonds. By testing adhesives, manufacturers can help to improve the performance and safety of their products.

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